The model created for the study combined data from field surveys of 51 spawning areas — which were conducted by partner organizations, including the Suquamish Tribe — with information about land use and climate. It predicted 10 to 40 percent of coho returning to spawn in many Kitsap streams will succumb to toxic runoff, while less than 10 percent will die in lightly developed areas at the south end of the county.

The model predicted much higher mortality rates in densely-developed areas of Seattle and Tacoma. Overall, the researchers found 40 percent of the area of Puget Sound river basins supporting coho are plagued by mortality rates that "substantively increase the risk of local population extinction."

Nathaniel Scholz at the Northwest Fisheries Science Center said busy roadways appeared to be a major factor in determining where pre-spawn mortality was highest.

“It’s the cars or traffic density that really separates it," Scholz said. "We see a big difference between local roads and highways.”

Scholz said the study can help planners identify where untreated stormwater from roads and parking lots poses a risk to salmon. Researchers hope to eventually forecast which established coho runs would be threatened by future development.

Scientists have studied the effects of stormwater pollution on coho for years but much is still not known. Researchers are trying to determine which of the thousands of compounds found in runoff are poisoning fish, how the fish are dying and why coho are more sensitive to the toxins than other salmon species, such as chum. Coho exposed to toxic stormwater become disoriented, lose equilibrium and die within a few hours.

Cleaning stormwater

Chris May with Kitsap County Public Works said the county is already incorporating those green stormwater systems into many new infrastructure projects and retrofitting older facilities where it can. The systems work by simulating the natural process for cleansing runoff. Water is circulated through a mix of organic soils and plantings to remove chemicals before being discharged.

“If we can mimic how wetlands and forests treat stormwater runoff in our urban areas we’ll be better off,” May said.

Recent green infrastructure projects include the Manchester stormwater park, a $4 million facility that treats runoff from more than 100 acres of property, and the Duwe’iq wetland near Clear Creek in Silverdale that filters runoff from 13 acres of parking lots and rooftops. Another project is planned next year at the former site of the Whispering Firs mobile home park off Ridgetop Boulevard.

County zoning now requires developers to use green stormwater systems unless they have a compelling reason not to. Homeowners and businesses can qualify for financial assistance to install green roofs and other small-scale stormwater filtration systems.

May said limited space and funding are the biggest challenges to building more stormwater treatment facilities. The county is relying on limited local funds to support the work.

“It’s a mixed bag of difficulties but we try to do our best,” he said.

Treating stormwater could help more than spawning coho. Scientists watch coho as an indicator of the health of water systems because they're particularly sensitive to environmental changes. Scholz said humans can improve habitat for a host of other species by treating stormwater.

“We’re focusing on coho but we’re trying to be thoughtful about the greater aquatic communities,” he said.